The invention relates to a method for the generation of a nuclear magnetic resonance (NMR)-spectrum from a measuring volume which is exposed to a highly homogeneous stationary magnetic field B.sub.0, wherein in a first run a radio frequency (RF)-pulse is irradiated into the measuring volume in order to excite the concerned nuclear spins, at least one magnetic gradient field G, for example for the selection of a certain slice from the measuring volume, is switched and after switching off the magnetic gradient field G a first data set is recorded which contains the observable measuring signals from the measuring volume.
A method of this type is known for example from DE-OS 32 09 263. It describes a method for the limitation of highly-resolved NMR spectrometry onto a selected area of a body, in which the area is prepared in that after the generation of a homogeneous magnetic field B.sub.0, magnetic gradient fields are superimposed the strengths of which vary in a linear manner in one spatial direction in each case, and subsequently the selected nuclear spins are selectively excited by means of corresponding RF-excitation pulses in a certain layer and respectively only in one certain volume element of the measuring volume. By this means a slice- or volume-selective spectrum can be recorded from the concerned measuring volume.
Owing to the fast switching of the magnetic gradient fields, correspondingly fast magnetic field variations are generated which lead on the one hand to forces acting upon the gradient coil because of the occurring Lorentz-forces and finally to mechanical vibrations of the gradient coil as well as of the construction parts of the NMR apparatus surrounding the measuring volume, and on the other hand also generate eddy currents in the surrounding conductive structures which themselves contribute to the magnetic field in the measuring volume. The eddy currents generated by the changes in the magnetic field which occur when switching the gradient fields, show in the beginning the same symmetry as the switched gradient fields so that, in the case of a completely symmetrical conductive environment, they should not contribute to the stationary magnetic field B.sub.0 owing to the total antisymmetry of the gradient fields with respect to the center of symmetry. Since, however, the conductive structures comprise normally of unsymmetries, finally also the stationary magnetic field B.sub.0 is impaired by the magnetic fields generated by the eddy currents.
The conductive structures, as for example copper pipes, radiation shields, electrical shields and the like, which are put into vibration by the acting forces described above owing to the gradient field switching, act in the stationary magnetic field B.sub.0, which is very strong for NMR applications, as generators which generate oscillating currents and thus oscillating magnetic fields, in particular oscillating contributions to the stationary magnetic field B.sub.0. Thus, the momentary value of the stationary magnetic field B.sub.0 is modulated.
As a result these disturbances of the homogeneous stationary magnetic field B.sub.0 cause so-called side bands in the recorded NMR spectrum and respectively artifacts in an NMR image composed thereof which can be attributed to side bands.